Visible light communications (VLC) technology provides for the exploration of light emitting diode (LED) luminaires to provide illumination and high capacity data transfer simultaneously. VLC is an advantageous form of data communication because it provides high data rates with low power requirements, has an unregulated bandwidth, has no interference with radio frequency (RF) systems, is low cost, and has no known health concerns. In VLC, the data wave form is modulated onto the instantaneous power of the optical carrier and an optical detector generates a current proportional to the received instantaneous power, often referred to as intensity modulation with direct detection (IM/DD).
Commercially available LED luminaires may be used to provide light to illuminate a space, such as a room or office, and the like, and provide the light needed for VLC. Dimming the LED luminaires is an essential feature in lighting to meet both functional and aesthetic requirements of a space. Dimming also conserves energy and expands the life span of the LED light sources used in the luminaire. The brightness, or dimming, of an LED may be adjusted by controlling the forward current through the LED. There are several known techniques to dim LEDs. These include analog dimming, digital dimming, and hybrid dimming. Analog dimming, also known as amplitude modulation (AM), or continuous current reduction, is a simple type of dimming control. This method reduces the current amplitude linearly to adjust the radiated optical flux. Digital dimming relies on the average duty cycle or signal density which represents the equivalent analog dimming level, e.g., a digitally modulated pulse train drives the LED at a constant current. Pulse-width modulation (PWM) is the simplest example of digital dimming. PWM relies on a fixed period of the PWM signal and adjusting the duty cycle proportionally to the required dimming percentage. PWM reduces light intensity more linearly and induces less of a chromaticity shift than AM. Hybrid dimming is a combination of AM and PWM which further reduces chromaticity shifts.
Optical orthogonal frequency division multiplexing (O-OFDM) is a promising parallel data transmission technique used in VLC where high data rates can be achieved by transmitting orthogonal subcarriers. O-OFDM systems do not require complex channel equalizers, the time varying channel can be easily estimated using frequency domain channel estimation, and adaptive modulation can be applied based on the up-link/down-link requested data rates.
Conventional systems and methods to combine O-OFDM signals with PWM dimming signals typically rely on superimposing O-OFDM signals onto the PWM dimming signal only while the PWM signal is high or during the on-state. Such a design is limited by the relatively low PWM line rate and results in a low capacity VLC link.
Thus, there is a need to combine the high data rates available with fast O-OFDM communication signals to provide a high capacity VLC link while maintaining a wide dimming range using commercially available LED luminaires which use relatively slow PWM dimming signals.